In the production of power from a system using a Rankine cycle, if the temperatures on the hot side from which the fluid expansion occurs are high enough, water is generally used as the working fluid in the cycle. Most of the heat sources available on the earth, however, are produced from low-grade energy which cannot efficiently produce a sufficiently high temperature to generate the pressures necessary to produce significant amounts of power in such a system. With water as the working fluid, sufficient pressures are not generated to efficiently operate a power-generating turbine. For this reason, organic fluids, which expand to a much higher pressure than water at the same working temperature, are advantageous for systems using thermodynamic Rankine cycles.